This invention relates to the transformation of image information for the accurate recordal, display and other processing of a color image. The transformation is particularly advantageous for the exacting reproduction of color image information on photographic film.
The invention further relates to the conversion of color-image information to a photographic record, with exacting control of color relative to a selected display.
The invention makes possible, for example, the preparation of a full-color photographic pre-press proof, with a single exposure, from page-makeup equipment as conventionally used in the graphic arts industry. The proof is of such exact quality, especially with regard to color, that it accurately predicts the appearance of an ink-on-paper printed reproduction of the same image information. Further, the invention enables the photographic proof to be produced with a high degree of automation, and hence with minimal operator involvement.
Photographic color media have heretofore often been deemed unacceptable for reproduction of color image information. A pre-press proof, for example, conventionally is prepared according to a relatively slow and costly Chromalin procedure with different black-and-white separations. Similarly, photographic media have often been deemed unacceptable for preparing an exacting record of color video image information, as commonly displayed on a television monitor.
It is understood that this perceived inadequacy of photographic media for proof quality and other exacting reproductions of color image information stems at least in part from the supposed inability of photographic color media to reproduce the complex, multiple-parameter color space of available printing inks and of other graphic display media.
There are other reasons why a print or other display of color image information stored in digital form may appear significantly different from the original scene. One is that the optical scanner or other input device which encodes the original scene, and the output device, as well as the printing or other display medium, e.g. television, do not necessarily process color values faithfully.
Various schemes have been employed to correct for degradation in prior digital color transformation of image information. U.S. Pat. No. 3,893,166 discloses a color correction system in which a computer transforms image information for graphic arts reproduction, after an operator has adjusted color-parameter controls of the computer. The article entitled "Pre-Press Picture Processing In The Graphic Arts Industry" by Peter C. Pugsley, IEEE Transactions on Communications, Volume CON-29, No. 12, December 1981, pp. 1891-1897, discusses color correction in the conversion of red-green-blue (RGB) input signals to cyan-magenta-yellow-black (CMYK) output signals. The conversion employs a multi-dimensional look-up table and a digital interpolator. The conversion data for the lookup table is computed from color parameters entered by the operator. European Patent Application Publication No. 0084228 (published July 27, 1983) discloses a system of nine look-up tables for modifying a three-color RGB input to a like output.
Aside from the noted media problem, the accurate transformation of color-image information to a selected display has been difficult in part because each color component influences other color components, and because transformation has required storing and processing the vast quantities of information in a color image. In particular, each pixel of a color image is commonly encoded into three or four constituent color components. Each color component is commonly defined with an eight-bit computer word and hence with one of two hundred and fifty-six values. Three components are common for the RGB additive color system, and four components are used for the CMYK subtractive color system. Television, for example, uses the additive system, whereas commercial printing commonly employs the subtractive system.
The color at any pixel of an image may be a function of all the color components. Hence, the values of three or four eight-bit bytes jointly specify the exact color of each pixel in an input image. There thus are either 256.sup.3 (sixteen million) possible input color values at a single pixel, or 256.sup.4 (four billion) such values. The output representation of the pixel involves a like number of values. Thus, a transformation for photographically printing color images obtained in a printing process may receive four input bytes, i.e. C, M, Y and K values, per pixel and produce three output bytes to define the R, G, and B values for that pixel.
The storage of conventional conversion data for this number of values exceeds the capabilities of present competitive commerical equipment. Further, storing a lesser, representative number of conversion values and interpolating between the representative values has required excessive computation time, even with a high speed main-frame computer. In addition, prior color correction of the kind illustrated by the first two publications noted above requires color perception and adjustment by an operator.
It accordingly is an object of this invention to provide a method and apparatus for the improved recordal of color-image information on a photographic medium with substantially exact control of color relative to a selected other display medium. A further object is to provide equipment that effects such image recordal with a high degree of automatic operation.
It is also an object of this invention to provide color-image transformation from one image-display medium to another which accounts for multiple medium-dependent artifacts and which yet can be practiced with competitive automated equipment.
It is a further object of the invention to provide a full multi-dimensional image transformation which produces each output parameter at a given pixel in response to all input parameters at that point. Information for an RGB image, for example, has three color components and hence has three such parameters.
It is also an object of the invention to provide a transformation of the above character in which the transformation data can readily be changed. This is desirable because an exacting transformation is based on calibrations of both the image-encoding input system and of the printing or other display output elements, e.g., recorder and film. The transformation of this invention accordingly can be changed when the properties of an input component or of an output component are altered.
A more particular object of the invention is to provide a color transformation of the above character that can be performed as part of a pre-press graphic arts operation. The transformation is to receive color-image input formation in a conventional manner with existing input equipment, and transform it for recordal with existing output equipment.
It further is an object of the invention to provide color transformation of the foregoing character that can readily be incorporated into existing graphic arts and other image-processing equipment.
Another object of the invention is to provide color transformation of the above character that can be practiced on a real-time basis with current commercial equipment.
Other objects of the invention will in part be obvious and will in part appear hereinafter.